1. Field of the Invention
The present invention generally relates to pressure transducers, and more particularly to an improved digital piezoresistive transducer in which stored digital data establishes and can be used to dynamically change the span of the transducer.
2. Description of the Prior Art
One class of pressure transducer, which is well known and widely used in industry, employs a number of piezoresistive strain responsive elements epitaxially formed on a semiconductor substrate with the piezoresistive elements interconnected in a bridge configuration. A pressure on the substrate unbalances the bridge, and the magnitude of the unbalance is a measure of the applied pressure.
In one widely used type of prior art transducer, the output terminals of the bridge are coupled to the input of an instrumentation amplifier, whose analog output voltage is a function of the magnitude of the differential input to the amplifier from the bridge. U.S. Pat. No. 4,765,188 ('188) issued to Roger Krechmery and Mark Finefrock and assigned to the assignee of this invention is an example of a prior art piezoresistive pressure transducer. In such transducers, an analog to digital convertor is used to convert the instrumentation amplifier output to a digital value.
While this type of prior art piezoresistive pressure transducers are generally satisfactory and are widely used, they have limitations. These limitations include a tendency of the instrumentation amplifier output and the bridge power supply output to drift, particularly in high temperature environments, degrading the accuracy of the transducer. Further, in these prior art transducers the span of the transducer cannot be readily adjusted over a wide range, or dynamically changed. Proposals have been made in the prior art to eliminate the instrumentation amplifier in bridge circuits generally by using a comparator and placing the bridge in the analog to digital feedback loop. But these proposals do not provide digital control of the transducer span and thus would require tight control of the manufacturing process for piezoresistive transducers. Further, there is no provision for dynamic range change.